This invention relates generally to an ultrasound colorflow imaging system and, more particularly, to methods and systems for scan sequencing in an ultrasound colorflow imaging system.
Ultrasound imaging is used in a variety of clinical settings, including, for example, obstetrics, gynecology, cardiology and oncology. Ultrasound imaging is also used to study anatomical structures, detect anomalies in tissues and measure blood flow within a body. In ultrasound imaging systems, a transducer probe generates and transmits acoustic waves and receives the echoes that are reflected, for example, by a body or a portion thereof.
Doppler ultrasound, which is based upon the Doppler effect, is used to measuring the rate of blood-flow through the human body, for example, through the heart, major arteries, and veins in the body. Doppler ultrasound works on the principle that the frequency of the reflected ultrasound pulses (echoes) that are reflected from a moving object is different from the frequency of the transmitted ultrasound pulses. The frequency of the echoes is higher than that of the transmitted ultrasound pulses if the object is moving towards the probe and vice versa. Doppler ultrasound measures the change in frequency of the echoes to calculate the flow velocity of a body fluid, such as blood. The change in the frequency of the echoes is also referred to as a Doppler shift.
One form of Doppler ultrasound is ultrasound colorflow imaging, in which a color is assigned to the direction of flow. For example, red color is assigned to a flow that is towards the transducer probe, and blue color is assigned to a flow that is away from the transducer probe. In ultrasound colorflow imaging, multiple ultrasound pulses are transmitted into the body that is to be examined. The Doppler shift between the ultrasound pulses is measured to provide an estimate of blood flow or velocity of a tissue. An ultrasonographer selects a velocity scale of the ultrasound pulses. The velocity scale determines a round trip travel time of the ultrasound pulse. The round trip travel time is the time taken by an ultrasound pulses to travel to the bottom of a colorflow region of interest (ROI) and back to the transducer probe for subsequent processing. The round trip travel time determines a pulse repetition frequency (PRF) of the ultrasound pulses in a packet that is formed from subsequent firing of the ultrasound pulses. PRF is the number of ultrasound pulses that are transmitted by the transducer probe in one second.
The scanning sequence of the ultrasound pulses is described in equation (1):V1 V1 V1 V2 V2 V2 V3 V3 V3 . . .  (1)
where V1 is a colorflow imaging vector that corresponds to an ultrasound pulse that is fired in direction 1,
V2 is a colorflow imaging vector that corresponds to an ultrasound pulse that is fired in direction 2,
V3 is a colorflow imaging vector that corresponds to an ultrasound pulse that is fired in direction 3, and
Where, for example, three colorflow imaging vectors being fired in the same direction constitute a packet.
When the velocity scale of the ultrasound pulses described in equation (1) is changed to a lower value, the PRF of the ultrasound pulses is reduced. As a result, the subsequent frame rate for formation of an ultrasound image is decreased. When the velocity scale of the ultrasound pulses described in equation (1) is changed to a higher value, the PRF of the ultrasound pulses is increased, and the round trip travel time for the ultrasound pulses is decreased. As a result, most of the time between transmissions of the ultrasound pulses is not used in the formation of the ultrasound image.
In order to more efficiently utilize the time between transmissions of the ultrasound pulses, and to maintain the frame rate simultaneously, the colorflow imaging vectors are interleaved. The interleaving of the colorflow imaging vectors is possible when a pulse repetition interval (PRI) of the colorflow imaging vector is at least two times greater than the round trip travel time of the colorflow imaging vectors. PRI is the time that is elapsed between the beginnings of emissions of two consecutive colorflow imaging vectors. The scanning sequence of the interleaved colorflow imaging vectors is described in equation (2):V1 V2 V3 V1 V2 V3 V1 V2 V3 . . .  (2)
As a result of interleaving, the packets for the three colorflow imaging vectors V1, V2, and V3 are fired at the same instant of time. In the scanning sequence as described in equation (2), the time elapsed between the adjacent fired colorflow imaging vector is greater than the round trip travel time of the colorflow imaging vectors. The PRF of the interleaved colorflow imaging vector (as described in equation (2)) is less than the PRF of colorflow imaging vector without interleaving (as described in equation (1)). The interleaving of colorflow imaging vectors therefore maintains the frame rate even though the PRF of the colorflow imaging vectors is reduced.
For interleaving of colorflow imaging vectors, an interleave factor is determined. The total number of colorflow imaging vectors are divided into interleave groups based on the interleave factor. However, the total number of colorflow imaging vectors, may not get evenly divided into interleave groups. As a result, the last interleave group may be shorter than the other interleave groups. Therefore, the last interleave group is appended with extra colorflow imaging vectors. However, it may happen that while displaying the ultrasound image, the extra colorflow imaging vectors, which are beyond the edge of the color region of interest (ROI) are not used in the formation of the ultrasound image. Further, time required in firing the extra colorflow imaging vectors, and the information that is acquired through firing of the extra colorflow imaging vectors gets wasted.